1. Field of the Invention
The present invention generally relates to a diagnostic circuit for use with emergency lighting. In particular, the present invention identifies failures in remotely located emergency lighting lampheads.
2. Description of the Prior Art
Emergency lighting is used in many types of facilities to provide DC battery-powered lighting in the event the main AC power supply is temporarily inoperative. Examples of these facilities include schools, hospitals, government offices, hotels, motels, industrial buildings, multiple unit dwellings, shopping malls, and airports. In many cases, these structures are very large and require that emergency lampheads be placed at several different locations to provide adequate coverage. Fire safety codes typically require that emergency lighting be tested periodically to ensure proper operation during an emergency. With a system employing many separate lampheads at scattered locations, these tests become labor intensive and time consuming.
Emergency lighting systems ordinarily include a battery to supply temporary power to one or more lampheads during an AC power loss, a charger for charging the battery from the AC power supply during standby operation, and a relay or other type of switching device for connecting the lampheads to the battery during loss of AC power. If diagnostic circuitry is provided in a central unit rather than in the remote lamphead, testing for proper battery and charger operation is facilitated. However, centrally located diagnostics make it difficult to check for proper operation of individual lampheads. Specifically, problems that can cause failure in particular lampheads include a defective, burned out, or improperly connected lamp or wire, which is difficult to detect from a central location.
In addition, remote lampheads that are connected to each other and the central battery and charging unit in a parallel “daisy chain” arrangement make it difficult for a diagnostic circuit located at the central unit to perform separate tests on each lamphead to identify the specific lamphead that requires service. Thus, if the central monitoring or diagnostic circuit merely shows that one of the lampheads is inoperable, but does not specify the identity or location of that lamphead, the system must be placed in emergency mode to visually identify the inoperable lamphead.
Determining operability at remote lampheads is also difficult since there is a need to minimize the number of lines between the remote lampheads and the central monitoring unit. Therefore, connecting additional wires between the remote lampheads and the central monitoring unit to support diagnostic functions adds to the cost of the system and increases the potential for additional failures. Further, the expense and complexity of diagnostic circuitry is ordinarily such that it is not practical to replicate the required circuitry at each remote lamphead.
Therefore, it is desirable to provide a diagnostic apparatus and method that can readily determine whether a failure exists in an emergency lighting system and can identify the particular lamphead that has failed.